Many communication systems modulate electromagnetic signals from baseband to higher frequencies for transmission, and subsequently demodulate those high frequencies back to their original frequency band at the receiver. The original (or baseband) signal may contain, for example: data, voice or video content These baseband signals may be produced by transducers such as microphones or video cameras, be computer generated, or be transferred from an electronic storage device. In general, the use of high frequencies provides longer range and higher capacity channels than baseband signals, and because high frequency signals can effectively propagate through the air, they can be used for wireless transmissions as well as hard-wired or wave-guided communications.
All of these signals are generally referred to as RF (radio frequency) signals, which are electromagnetic signals; that is, waveforms with electrical and magnetic properties within the electromagnetic spectrum normally associated with radio wave propagation.
Wired communication systems which employ such modulation and demodulation techniques include computer communication systems such as local area networks (LANs), point-to-point communications, and wide area networks (WANs) such as the internet. These networks generally communicate data signals over electrically conductive or optical fibre channels. Wireless communication systems which may employ modulation and demodulation include those for public broadcasting such as AM and FM radio, and UHF and VHF television. Private wireless communication systems may include cellular telephone networks, personal paging devices, HF radio systems used by taxi services, microwave backbone networks, interconnected appliances under the Bluetooth standard, and satellite communications. Other wired and wireless systems which use RF modulation and demodulation would be known to those skilled in the art.
The focus of this document is on down-conversion or demodulation; the conversion of high frequency signals to lower frequency levels. In the case of a wireless RF receiver, for example, demodulation would typically consist of down-converting a received signal from its carrier frequency to baseband.
Most RF receivers use the “super-heterodyne” topology for down-conversion, which provides good performance in a limited scope of applications, such as in public-broadcast FM radio receivers. As will be explained, the super-heterodyne's limitations make its use in more sophisticated modern applications expensive, and its performance poor.
The super-heterodyne receiver uses a two-step frequency translation method to convert an RF signal to a baseband signal. FIG. 1 presents a block diagram of a typical super-heterodyne receiver 10. The mixers labelled M1 12 and M2 14 perform the task of translating the RF signal to baseband, while the balance of the components amplify the signal being processed and filter noise from it.
The RF band pass filter (BPF1) 18 first filters the signal coming from the antenna 20 (note that this band pass filter 18 may also be a duplexer). A low noise amplifier 22 then amplifies the filtered antenna signal, increasing the strength of the RF signal and reducing the noise figure of the receiver 10. The signal is next filtered by another band pass filter (BPF2) 24 usually identified as an image rejection filter. The signal then enters mixer M1 12 which multiplies the signal from the image rejection filter 24 with a periodic signal generated by the local oscillator (LO1) 26. The mixer M1 12 receives the signal from the image rejection filter 24 and translates it to a lower frequency, known as the first intermediate frequency (IF1).
Generally, a mixer (such as M1 12 or M2 14) is a circuit or device that accepts as its input two different frequencies and presents at its output:    (a) a signal equal in frequency to the sum of the frequencies of the input signals;    (b) a signal equal in frequency to the difference between the frequencies of the input signals; and    (c) the original input frequencies.The typical embodiment of a mixer is a digital switch which may generate significantly more tones than those stated above.
The IF1 signal is next filtered by a band pass filter (BPF3) 28 typically called the channel filter, which is centred around the IF1 frequency, thus filtering out the unwanted products of the first mixing processes; signals (a) and (c) above. This is necessary to prevent these signals from interfering with the desired signal when the second mixing process is performed.
The signal is then amplified by an intermediate frequency amplifier (IFA) 30, and is mixed with a second local oscillator signal using mixer M2 14 and local oscillator (LO2) 32. The second local oscillator LO2 32 generates a periodic signal which is typically tuned to the IF1 frequency. Thus, the signal coming from the output of M2 14 is now at baseband, that is, the frequency at which the signal was originally generated. Noise is now filtered from the desired signal using the low pass filter LPF 38, and the signal is passed on to some manner of presentation, processing or recording device. In the case of a radio receiver, this might be an audio amplifier and speaker, while in the case of a computer modem this may be an analogue-to-digital convertor.
Note that the same process can be used to modulate or demodulate any electrical signal from one frequency to another.
The main problems with the super-heterodyne design are:                it requires expensive off-chip components, particularly band pass filters 18, 24, 28, and low pass filter 38;        the off-chip components require design trade-offs that increase power consumption and reduce system gain;        image rejection is limited by the off-chip components, not by the target integration technology;        isolation from digital noise can be a problem; and        it is not fully integratable.        
The band pass and low pass filters 18, 24, 28 and 38 used in super-heterodyne systems must be high quality devices, so electronically tunable filters cannot be used. As well, the only way to use the super-heterodyne system in a multi-standard/multi-frequency application is to use a separate set of off-chip filters for each frequency band.
Direct-conversion topologies attempt to perform down-conversion in a single step, using one mixer and one local oscillator. In the case of down-conversion to baseband, this requires a local oscillator (LO) with a frequency equal to the carrier frequency of the input RF signal.
However, this technique will generate DC noise signals which interfere with low-frequency information contained in the demodulated baseband signal. These DC noise signals are particularly difficult to overcome because they are typically unpredictable and time-varying. Several mechanisms which may generate such DC noise signals in direct-conversion topologies include the following:                1. local oscillator leakage. Local oscillator (LO) power leaking to the RF input will result in DC levels at the mixer output because it will be mixed with itself. Because one of the output signals from a mixer is the difference between the two frequencies being mixed together, and the LO is generating a powerful signal at the same frequency as the carrier frequency of the incoming signal being demodulated, the LO signal itself is demodulated to generate a DC signal at the mixer output;        2. leakage of channel interferers. DC levels may be created at the mixer output when large nearby RF signals leak into the local oscillator port of the mixer and are self-mixed down to DC;        3. offsets due to mismatching in devices on a fully-integrated implementation;        4. 1/f noise at baseband. 1/f noise is noise with a power spectra that is inversely proportional to the frequency—in other words, the power of the noise signal is greater close to DC (direct current). 1/f noise, or “flicker noise” is generated largely by the charge trapping and de-trapping properties of MOSFETs; and        5. intermodulation products. Mixing generates sum and difference products from primary signals. Intermodulation products are distortions of those products, which may be generated by non-linearities in electronic components, or harmonics in the signals being mixed.Hence, there is a potential for large, time-varying DC signals to interfere with the comparatively low-amplitude signals of interest, at or near DC, at the output of the demodulator.        
A number of attempts have been made to reduce or compensate for the level of these DC noise signals, but none have been very effective or practical:    1. Capacitive Coupling            Placing a capacitor in series with the signal path will block DC noise signals but will also block components of the desired signal near zero frequency.        The severity of the data loss is dependent upon the transmission modulation and signal coding.        Capacitive coupling also has the disadvantage that the size of the capacitors are generally too large for a fully integrated receiver.            2. Adaptive Feedback            DC noise signals may also be removed by the use of adaptive feedback that time-averages the suspected DC offset value and subtracts the corresponding amount from a convenient point along the receive path. While feedback-based DC-offset reduction techniques are more effective than capacitive coupling and are more easily applied to integrated solutions, the following must be considered when they are applied:        a. the increased level of complexity they add to the design;        b. since the DC offsets and near DC offsets may be indistinguishable from the desired data, some amount of training time is normally required on a periodic basis to determine the DC offset accurately; and        c. if a long-term average of the DC offset is used to estimate how much offset must be subtracted from the input, then this technique will not respond well to rapid variations in the DC offset level; and            3. Good Matching of Devices            Mis-matching of transistors causes noise and adversely affects performance.        The degree of mis-matching increases as component sizes decrease, so performance and yields drop with highly integrated applications. Typically, this problem is addressed by using large device sizes and/or using multiple components in parallel. Neither of these methods are highly effective and of course, result in larger components.Thus, none of the currently used techniques for addressing the DC noise problem in direct-conversion architectures is particularly effective.        
It is also of note that the continuing desire to implement low-cost, power efficient receivers has led to intensive research into the use of highly integrated designs, an increasingly important aspect for portable systems, including cellular telephone handsets. This has proven especially challenging as the frequencies of interest in the wireless telecommunications industry (especially low-power cellular/micro-cellular voice/data personal communications systems) have risen above those used previously (approximately 900 MHz) into the spectrum above 1 GHz.
Thus, there is a need for a method and apparatus for demodulation which addresses the problems above. It is desirable that this design be fully-integratable, inexpensive and high performance. As well, it is desirable that this design be easily applied to multi-standard/multi-frequency applications.